Apparatus for controlling a plurality of hydraulic motors and clutch

ABSTRACT

An apparatus for controlling a plurality of hydraulic motors and a clutch is capable of reliably implementing a sequence for engaging and disengaging the clutch and for fixing and clearing a zero tilt rotation amounts of the hydraulic motors, thereby preventing a speed change shock or load slip in the hydraulic motors. The apparatus includes zero tilt rotation fixing means ( 10 A), a clutch ( 5 ), hydraulic vehicle speed detecting means ( 32 A), and control valve means ( 30 A) that releases an output command pressure to a return pressure connected to a tank ( 19 ) until a vehicle speed signal pressure to be received reaches a start pressure of a predetermined value, and begins to output the command pressure to the zero tilt rotation fixing means and the clutch when the vehicle speed signal pressure exceeds a predetermined value.

This is a division of application Ser. No. 09/814,001, filed on Mar. 22,2001, now U.S. Pat. No. 6,622,594.

FIELD OF THE INVENTION

The present invention relates to an apparatus for controlling aplurality of hydraulic motors and a clutch and, more particularly to, anapparatus for controlling a plurality of hydraulic motors and a clutchthat is adapted to output the output torque of the plurality ofhydraulic motors by connecting the output torque via the clutch in ahydraulic drive unit for a working machine, such as a wheel loader, ahydraulic excavator, or the like.

BACKGROUND OF THE INVENTION

Hitherto, in a hydraulic travel drive unit for a vehicle driven byconnecting the output torque of a plurality of hydraulic motors throughthe intermediary of a clutch, the output torque, vehicle speed, and thelike are controlled by connecting and disconnecting the clutch when avehicle speed reaches a predetermined value.

FIG. 6 is a control circuit diagram of a control apparatus for aplurality of hydraulic motors and a clutch of a conventional hydraulictravel drive unit. The control circuit is constituted primarily by ahydraulic pump 50, a first hydraulic motor 51, a second hydraulic motor52, a clutch 53, and a vehicle speed detection pump 54. The firsthydraulic motor 51 and the second hydraulic motor 52 are connected inparallel to the hydraulic pump 50 driven by an engine 15, and are drivenby the discharge pressure oil of the hydraulic pump 50. A motor gear 51b is fixedly provided on a first output shaft 51 a of the firsthydraulic 51, and the motor gear 51 b is in mesh with a driving shaftgear 55 a of a driving shaft 55 for driving a vehicle. The output torqueof the first hydraulic motor 51 is always transmitted to the drivingshaft 55 via the motor gear 51 b and the driving shaft gear 55 a. Awheel 70 is installed on a shaft end of the driving shaft 55.

A clutch 53 is provided on a second output shaft 52 a of the secondhydraulic motor 52. A second motor gear 53 b is fixedly provided on athird output shaft 53 a of the clutch 53, and the second motor gear 53 bis in mesh with the driving shaft gear 55 a of the driving shaft 55 fordriving a vehicle. The clutch 53 has a spring 72 therein. The secondoutput shaft 52 a and the third output shaft 53 a are engaged at asurface S by the spring 72 when no oil pressure is being supplied to anoil chamber 73. When oil pressure is supplied to the oil chamber 73, anoil pressure force overcomes the urging force of the spring 72, causingthe surface S to separate thereby to disengage the second output shaft52 a and the third output shaft 53 a.

The output torque of the second hydraulic motor 52 is transmitted to thedriving shaft 55 for driving the vehicle through the intermediary of theclutch 53, the second motor gear 53 b, and the driving shaft g ar 55 awhen the clutch 53 is in mesh. A rod distal end of a first cylinder 62controlled by a first servo valve 61 is attached to one end of a firstswash plate 65 of the first hydraulic motor 51. Furthermore, a roddistal end of a second cylinder 64 controlled by a second servo valve 63is attached to one end of a second swash plate 66 of the secondhydraulic motor 52.

The vehicle speed detection pump 54 is connected to the driving shaft55. The discharge oil of the vehicle speed detection pump 54 is drainedinto a tank 71 through a throttle 32 c. The discharge port of thevehicle speed detection pump 54 is connected to a pressure receivingportion of a tilt rotation fixing control valve 58 and a pressurereceiving portion of a clutch switching valve 59. The oil introducedfrom the tank 71 and discharged from a control pump 56 is set at aconstant oil pressure by a relief valve 57, and supplied to port P1 ofthe tilt rotation fixing control vale 58. Port P2 of a tilt rotationfixing valve 58 is connected to a port P3 of the clutch switching valve59. A port P4 of the clutch switching valve 59 is connected to the oilchamber 73 of the clutch 53, and a port P5 is connected to the tank 71.The port P2 of the tilt rotation fixing control valve 58 is connected tothe pressure receiving portion of a zero tilt rotation fixing valve 60.The drive pressure for driving the second hydraulic motor 52 is suppliedto a port P6 of the zero tilt rotation fixing valve 60, and a port P7thereof is connected to the second servo valve 63.

FIG. 6 illustrates a state of the control circuit when an acceleratorpedal is depressed to start acceleration. More specifically, since thevehicle speed is zero, so that the oil pressure output from the vehiclespeed detection pump 54 is zero, and both the tilt rotation fixingcontrol valve 58 and the clutch switching valve 59 are set at position“a” by the urging forces of springs 67 and 68. The zero tilt rotationfixing valve 60 is also set at position “a” the oil pressure in the oilchamber 73 is zero, so that the clutch 53 is engaged at surface S by theurging force of the spring 72.

When the acceleration is begun, the first cylinder 62 and the secondcylinder 64 are extended and retracted in response to the commands fromthe first servo valve 61 and the second servo valve 63, causing thefirst swash plate 65 and the second swash plate 66 of the firsthydraulic motor 51 and the second hydraulic motor 52, respectively, tobe at their maximum tilts.

FIG. 7 shows a relationship between discharge capacity D (cc/rev)indicating a tilt rotation amount and vehicle speed V. The drivepressure of the second hydraulic motor 52 that decreases as vehiclespeed V increases acts on the second servo valve 63 to conduct controlso as to reduce the tilt rotation amount of the second swash plate 66along curve A shown in FIG. 7. As the vehicle speed increases, thedischarge volume of the vehicle speed detection pump 54 increases, andthe oil pressure on the upstream side from the throttle 32 c alsoincreases. When the second swash plate 66 of the second hydraulic motor52 reaches an approximately zero tilt rotation amount as the vehiclespeed increases, that is, when vehicle speed V1 is reached in FIG. 7,the tilt rotation fixing control valve 58 is switched to position “b”.This causes the constant oil pressure output by the control pump 56through the port P2 of the tilt rotation fixing control valve 58 to besupplied to the pressure receiving portion of the zero tilt rotationfixing valve 60, so that the zero tilt rotation fixing valve 60overcomes the urging force of the spring 69 and acts at position “b”.Thus, the drive pressure of the second hydraulic motor 52 is supplied tothe second servo valve 63 through the port P7 of the zero tilt rotationfixing valve 60. Based on the supplied drive pressure, the second servovalve 63 outputs a command for fixing the position of the secondcylinder 64 to the second cylinder 64 thereby to fix the tilt rotationamount of the second swash plate 66. This means that the second swashplate 66 is fixed to the zero tilt rotation amount.

The vehicle speed continues to increase after the second swash plate 66of the second hydraulic motor 52 is fixed to the zero tilt rotationamount; hence, the oil pressure output by the vehicle speed detectionpump 54 continues to increase, and the clutch switching valve 59 acts atposition “b” when vehicle speed V2 is reached. Thus, the constant oilpressure output by the control pump 56 is supplied to the oil chamber 73of the clutch 53 through the ports P1 and P2 of the tilt rotation fixingcontrol valve 58 and the ports P3 and P4 of the clutch switching valve59. The surface S of the clutch 53 is separated to clear the engagement;therefore, the vehicle is driven only by the first hydraulic motor 51thereafter.

Meanwhile, in the first hydraulic motor 51, the first swash plate 65that has been fixed at the maximum tilt rotation amount is released fromthe maximum tilt rotation amount by an oil pressure signal (not shown)issued when the tilt rotation fixing control valve 58 is switched toposition “b”. Then, the tilt rotation amount of the first swash plate 65decreases according to the drive pressure of the first hydraulic motor51 that reduces as the vehicle speed increases from vehicle speed V1.

To increase the speed by such a hydraulic travel drive unit, the tiltrotation fixing control valve 58 is first operated at position “b” whenthe tilt rotation amount of the second swash plate 66 of the secondhydraulic motor 52 reaches approximately zero, and the second swashplate 66 of the second hydraulic motor 52 is fixed to the zero tiltrotation amount, then the clutch switching valve 59 is operated atposition “b” to disengage the clutch 53. To reduce the speed, the clutchswitching valve 59 is first operated at position “a” to engage theclutch 53, then the tilt rotation fixing control vale 58 is operated atposition “a” to clear the zero tilt rotation amount of the second swashplate 66 of the second hydraulic motor 52.

However, the conventional hydraulic travel drive unit described aboveposes the following problem.

The area of the pressure receiving portion of the tilt rotation fixingcontrol valve 58 and the spring 67 are set such that the tilt rotationfixing control valve 58 is actuated at position “a” when the vehiclespeed is smaller than vehicle speed V1, and at position “b” when thevehicle speed is the vehicle speed V1 or more. Similarly, the area ofthe pressure receiving portion of the clutch switching valve 59 and thespring 68 are set such that the clutch switching valve 59 is actuated atposition “a” when the vehicle speed is larger than V1 but smaller thanV2, and at position “b” when the vehicle speed is V2 or more. However,due to changes in frictional resistance forces of valve spools caused byoil temperature, an increase in leakage due to time-dependent changes ofvalve spool diameters, or for other reasons, there are cases where asequence is unsuccessful when the vehicle speed is increased. In thesequence, the second swash plate 66 is first fixed to the zero tiltrotation amount, then the clutch 53 is disengaged to release it. Forinstance, the clutch 53 may be released before the second swash plate 66reaches the zero tilt rotation amount. If this happens, there will be aspeed change shock when the clutch 53 is released. This causes thesecond hydraulic motor 52 to race, and all the engine driving force willbe undesirably supplied to the second hydraulic motor 52, resulting inno load on the first hydraulic motor 51.

In a normal decelerating operation, the tilt rotation amount of thesecond swash plate 66 changes from the zero tilt rotation amount afterthe clutch 53 is engaged. If, however, the tilt rotation amount of thesecond swash plate 66 starts to change with the clutch 53 stillreleased, then the same problem as that in the accelerating operationarises.

SUMMARY OF THE INVENTION

The present invention has been made with a view toward solving theproblem described above, and it is an object of the present invention toprovide an apparatus for controlling a plurality of hydraulic motors anda clutch that is capable of reliably implementing a sequence forengaging and disengaging a clutch and for fixing and clearing the zerotilt rotation amount of the hydraulic motors thereby to prevent a speedchange shock or load slip in the hydraulic motors.

To this end, according to a first aspect of the present invention, thereis provided an apparatus for controlling a plurality of hydraulic motorsand a clutch in which a single driving shaft is driven by outputs of aplurality of hydraulic motors, and one of the plurality of hydraulicmotors drives the driving shaft through the clutch, the apparatusincluding zero tilt rotation fixing means for fixing the tilt rotationamount of a first hydraulic motor to zero when a zero-fixing pressure ofa predetermined value is input, a clutch that is disengaged when arelease pressure of a predetermined value that is larger than thezero-fixing pressure is input, hydraulic vehicle speed detecting meansfor detecting a vehicle speed by a vehicle speed signal pressure basedon a vehicle speed, and control valve means that releases an outputcommand pressure to a return pressure connected to a tank until avehicle speed signal pressure received from the hydraulic vehicle speeddetecting means reaches a start pressure of a predetermined value, andbegins to output the command pressure to the zero tilt rotation fixingmeans and the clutch when the vehicle speed signal pressure becomeslarger than a predetermined value.

With this arrangement, the vehicle speed signal pressure based on avehicle speed is detected by the hydraulic vehicle speed detectingmeans, and the command pressure output by the control valve means isreleased to the return pressure if the detected vehicle speed signalpressure is the start pressure or less of the predetermined value. Whenthe vehicle speed signal pressure becomes larger than the startpressure, the control valve means starts to output a command pressure ofa magnitude based on the vehicle speed signal pressure to the zero tiltrotation fixing means and the clutch. While no oil pressure is beingsupplied to an oil chamber of the clutch, a spring for retaining theengagement of the clutch is set such that the clutch is disengaged inresponse to a command pressure that is larger than a signal pressure forfixing the tilt rotation amount of the first motor to a zero tiltrotation amount by the zero tilt rotation fixing means. Thus, in theacceleration mode, the zero tilt rotation amount is obtained first, thenthe clutch is disengaged thereafter. This permits reliableimplementation of a sequence for engaging and disengaging a clutch andfor fixing and clearing the zero tilt rotation amount of the hydraulicmotors at the time of acceleration, thereby making it possible toprevent a speed change shock or load slip in hydraulic motors.

According to a second aspect of the present invention, there is providedan apparatus for controlling a plurality of hydraulic motors and aclutch in which a single driving shaft is driven by outputs of aplurality of hydraulic motors, and one of the plurality of hydraulicmotors drives the driving shaft through the clutch, the apparatusincluding a first servo valve that controls the tilt rotation amount ofa first hydraulic motor and sets the tilt rotation amount of the firsthydraulic motor to a zero tilt rotation amount when a zero fixingpressure of a predetermined value is received, a clutch that isdisengaged when a release pressure of a predetermined value that islarger than the zero fixing pressure is received, hydraulic vehiclespeed detecting means for detecting the vehicle speed at a vehicle speedsignal pressure based on a vehicle speed, and control valve means thatreleases an output command pressure at return pressure connected to atank until the vehicle speed signal pressure received from the hydraulicvehicle speed detecting means reaches a start pressure of apredetermined value, and begins to output the command pressure to thefirst servo valve and the clutch when the vehicle speed signal pressureexceeds a predetermined value.

With this arrangement, the vehicle speed signal pressure based on avehicle speed is detected by the hydraulic vehicle speed detectingmeans, and the command pressure output by the control valve means isreleased at the return pressure as long as a detected vehicle speedsignal pressure remains at the start pressure of the predetermined valueor less. If the vehicle speed signal pressure exceeds the startpressure, then the control valve means outputs a command pressure of amagnitude based on a vehicle speed signal pressure to the first servovalve and the clutch. The spring for retaining the engagement of theclutch while no oil pressure is being supplied to the oil chamber of theclutch is set such that the clutch is released when the signal pressureexceeds the signal pressure for fixing the tilt rotation amount of thefirst motor to the zero tilt rotation amount by the first servo valve.Therefore, the zero tilt rotation amount is reached first, then theclutch is released at the time of acceleration. At the time ofdeceleration, the clutch is first engaged, then the zero tilt rotationamount is cleared. Thus, the sequence for engaging and disengaging theclutch and for fixing and clearing the zero tilt rotation amount of ahydraulic motor can be securely implemented at all times, making itpossible to prevent a speed change shock or load slip in a hydraulicmotor.

According to a third aspect of the present invention, there is providedan apparatus for controlling a plurality of hydraulic motors and aclutch in which a single driving shaft is driven by outputs of aplurality of hydraulic motors, and one of the plurality of hydraulicmotors drives the driving shaft through the clutch, the apparatusincluding a first servo valve that controls the tilt rotation amount ofa first hydraulic motor and sets the tilt rotation amount of the firsthydraulic motor to a zero tilt rotation amount when a zero fixingpressure of a predetermined value is received, a zero tilt rotationdetection valve that detects the tilt rotation amount of the firsthydraulic motor and applies a command pressure to the clutch thereby todisengage the clutch when the detected tilt rotation amount is zero,hydraulic vehicle speed detecting means for detecting the vehicle speedat a vehicle speed signal pressure based on a vehicle speed, and controlvalve means that releases an output command pressure to a returnpressure connected to a tank until the vehicle speed signal pressurereceived from the hydraulic vehicle speed detecting means reaches astart pressure of a predetermined value, and begins to output thecommand pressure to the first servo valve and the zero tilt rotationdetection valve when the vehicle speed signal pressure exceeds apredetermined value.

With this arrangement, the vehicle speed signal pressure based on avehicle speed is detected by the hydraulic vehicle speed detectingmeans, and the command pressure output by the control valve means isreleased at the return pressure as long as a detected vehicle speedsignal pressure remains at the start pressure of the predetermined valueor less. If the vehicle speed signal pressure exceeds the startpressure, then the control valve means outputs a command pressure of amagnitude based on a vehicle speed signal pressure to the first servovalve and the zero tilt rotation detection valve. At the time ofacceleration, the first servo valve fixes the first hydraulic motor to azero tilt rotation amount when the command pressure reaches apredetermined value. When the zero tilt rotation detection valve detectsthat the first hydraulic motor has been fixed to zero tilt rotationamount, it connects the command pressure to the oil chamber of theclutch to release the clutch. This ensures that the first hydraulicmotor is always fixed to the zero tilt rotation amount first, then theclutch is released. At the time of deceleration, the command pressure tothe first servo valve is first shut off, so that the fixed zero tiltrotation amount is cleared. After the zero tilt rotation detection valvedetects that the fixed zero tilt rotation amount has been cleared, theoil chamber of the clutch is placed in communication with returnpressure Pt, causing the clutch to be engaged. Thus, the sequence forengaging and disengaging the clutch and for fixing and clearing the zerotilt rotation amount of a hydraulic motor can be securely implemented atall times, making it possible to prevent a speed change shock or loadslip in a hydraulic motor.

According to a fourth aspect of the present invention, there is providedan apparatus for controlling a plurality of hydraulic motors and aclutch in which a single driving shaft is driven by outputs of aplurality of hydraulic motors, and one of the plurality of hydraulicmotors drives the driving shaft through the clutch, the apparatusincluding zero tilt rotation fixing means for fixing the tilt rotationamount of a first hydraulic motor to zero when a zero fixing pressure ofa predetermined value is received, a clutch that is released when arelease pressure of a predetermined value that is larger than the zerofixing pressure is received, hydraulic vehicle speed detecting means fordetecting vehicle speed at a vehicle speed signal pressure based on avehicle speed, and control valve means that applies an output commandpressure to the zero tilt rotation fixing means and the clutch when thevehicle speed signal pressure received from the hydraulic vehicle speeddetecting means is larger than a predetermined value, while it begins torelease the command pressure to a return pressure connected to a tankwhen the vehicle speed signal pressure becomes smaller than thepredetermined value.

With this arrangement, the vehicle speed signal pressure based on avehicle speed is detected by the hydraulic vehicle speed detectingmeans. If the detected vehicle speed signal pressure is larger than astart pressure of a predetermined value, then the control valve meanssupplies a command pressure of a magnitude based on a vehicle speedsignal pressure to the zero tilt rotation fixing means and the clutch.If the vehicle speed signal pressure reduces to the start pressure ofthe predetermined value or less, then the control valve means releasesthe command pressure to the return pressure connected to the tank. Theurging force of a spring for retaining the engagement of the clutchwhile no oil pressure is being supplied to the oil chamber of the clutchis set such that the clutch is disengaged in response to a commandpressure that is larger than a signal pressure for fixing the tiltrotation amount of the first motor to a zero tilt rotation amount by thezero tilt rotation fixing means. Thus, the clutch is engaged first, thenthe zero tilt rotation amount is disengaged at the time of deceleration.This permits reliable implementation of a sequence for engaging anddisengaging a clutch and for fixing and clearing the zero tilt rotationamount of the hydraulic motors at the time of deceleration, therebymaking it possible to prevent a speed change shock or load slip in ahydraulic motor.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a control circuit diagram of an apparatus for controlling aplurality of hydraulic motors and a clutch according to a firstembodiment of the present invention;

FIG. 2A is a diagram showing a relationship between the dischargevolumes of hydraulic motors shown in FIG. 1 and vehicle speed, and FIG.2B is a diagram showing a relationship between valves and vehicle speed;

FIG. 3A is a schematic representation illustrating a sequence forengaging and disengaging a clutch and for fixing and clearing a zerotilt rotation amount of the hydraulic motors at the time ofacceleration, and FIG. 3B is a schematic representation illustrating asequence for engaging and disengaging the clutch and for fixing andclearing a zero tilt rotation amount of the hydraulic motors at the timeof deceleration;

FIG. 4 is a control circuit diagram according to a second embodiment ofthe present invention;

FIG. 5 is a control circuit diagram according to a third embodiment ofthe present invention;

FIG. 6 is a diagram showing a conventional control circuit of anapparatus for controlling a plurality of hydraulic motors and a clutch;and

FIG. 7 is a diagram illustrating a relationship between dischargevolumes and vehicle speed in the control circuit shown in FIG. 6.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

A first embodiment in accordance with the present invention will bedescribed in conjunction with FIG. 1 through FIG. 3.

Referring to FIG. 1, the construction of the first embodiment will bedescribed. The discharge oil of a hydraulic pump 16 driven by an engine15 is connected in parallel to a variable-capacity type first hydraulicmotor 1 and a variable-capacity type second hydraulic motor 2 throughthe intermediary of a first main circuit 17 and a second main circuit18. The output shaft of the second hydraulic motor 2 is always inconnection, via a reduction gear 4, with a driving shaft 3 for driving awheel 70 of a vehicle, and the output shaft of the first hydraulic motor1 is coupled to the driving shaft 3 through the intermediary of a clutch5 and the reduction gear 4.

The clutch 5 has a spring 72 therein, so that it is engaged at surface Sby the spring 72 while no pressure oil is being supplied to an oilchamber 73. When pressure oil is supplied to the oil chamber 73, the oilpressure overcomes the urging force of the spring 72 to cause surface Sto be detached, thereby disengaging the clutch 5.

The drive pressure oil of the first main circuit 17 or the second maincircuit 18 is connected to a rod chamber 7 a of a first tilt rotationcylinder 7 for controlling the tilt rotation amount of the firsthydraulic motor 1 via an FR switching valve 6 that is switched bypressure oil from a back-and-forth motion operating valve (hereinafterreferred to as the “FR operating valve”), which is not shown, a firstpilot pressure receiving portion 8 a of a first servo valve 8, and portP of the first servo valve 8. Port A of the first servo valve 8 isconnected to a bottom chamber 7 b of the first tilt rotation cylinder 7.

The vehicle speed has Hi mode and Lo mode. When the zero tilt rotationfixing valve 11A for restricting the permissible rotational speed in theHi mode is in a shut-off position, variable control pressure Pmproportional to the engine speed supplied from the FR switching valve 6is supplied to a second pilot pressure receiving portion 8 b that islarger than the first pilot pressure receiving portion 8 a of the firstservo valve 8 through a shuttle valve 12. The first servo valve 8supplies the pressure oil, the pressure of which has been reduced fromport P to port A, to the bottom chamber 7 b of the first tilt rotationcylinder 7 according to the position where the spring force of a spring8 c is balanced with the urging force provided by the drive pressure oilof the first pilot pressure receiving portion 8 a and the controlpressure of the second pilot pressure receiving portion 8 b. This is howthe tilt rotation amount of the first hydraulic motor 1 is controlled.

The high-pressure oil of the first main circuit 17 or the second maincircuit 18 that is supplied through the shuttle valve 9 to port P of asmall tilt rotation fixing valve 13A for restricting the permissiblerotational speed in the Lo mode. Port A of the small tilt rotation valve13A is connected to a bottom chamber of a stopper cylinder 14, and a rodchamber of the stopper cylinder 14 is connected to a tank 19. When thesmall tilt rotation fixing valve 13A is in the position where port P andport A are in communication, a piston rod 14 a of the stopper cylinder14 moves to an extension end, thereby setting the first hydraulic motor1 to a small tilt rotation amount position. When the small tilt rotationfixing valve 13A is in the position where the communication between portP and port A is shut off, the piston rod 14 a of the stopper cylinder 14can move to a contraction end by an external force, thereby setting thefirst hydraulic motor 1 to a zero tilt rotation amount position. This isthe Lo mode wherein the small tilt rotation fixing valve 13A is set inthe position where port P and port A are in communication, so that thefirst hydraulic motor 1 can reduce its tilt rotation amount to the smalltilt rotation amount position. In the Hi mode, the small tilt rotationfixing valve 13A is set in the position where the communication betweenport P and port A is shut off, so that the first hydraulic motor 1 canreduce its tilt rotation amount to nearly the zero tilt rotation amountposition.

When the zero tilt rotation fixing valve 11A is in the communicatingposition, the high-pressure oil of the first main circuit 17 or thesecond main circuit 18 supplied through the zero tilt rotation fixingvalve 11A from the shuttle valve 9 overrides variable control pressurePm and is supplied to the second pilot pressure receiving portion 8 b ofthe first servo valve 8 through the shuttle valve 12. This causes thefirst servo valve 8 to have its maximum opening by the communicationestablished between port P and port A. As a result, the piston rod 7 cof the first tilt rotation cylinder 7 to move to the left by thepressure oil supplied to the bottom chamber 7 b from the first servovalve 8. Then, at the small tilt rotation amount position or the nearlyzero tilt rotation amount position of the first hydraulic motor 1 set bythe piston rod 14 a, a stopper 7 d abuts against the piston rod 14 a viathe piston rod 7 c so as to fix the first hydraulic motor 1 at the smalltilt rotation amount position or the nearly zero tilt rotation amountposition. Thus, the zero tilt rotation fixing valve 11A, the first servovalve 8, the first tilt rotation cylinder 7, the stopper 7 d secured tothe first tilt rotation cylinder 7, the small tilt rotation fixing valve13A, and the stopper cylinder 14 make up the zero tilt rotation fixingmeans 10A for fixing the tilt rotation amount of the first hydraulicmotor 1.

Meanwhile, the pressure oil of the first main circuit 17 or the secondmain circuit 18 supplied through a shuttle valve 29 is connected to arod chamber 27 a of a second tilt rotation cylinder 27 for controllingthe tilt rotation amount of the second hydraulic motor 2, the firstpilot pressure receiving portion 28 a of a second servo valve 28, andport P of the second servo valve 28. Port A of the second servo valve 28is connected to the bottom chamber 27 b of the second tilt rotationcylinder 27.

A hydraulic vehicle speed detecting means 32A is equipped with a vehiclespeed detection pump 32 a driven by the reduction gear 4, a highpressure selector valve 32 b for selecting vehicle speed signal pressurePv discharged by the vehicle speed detection pump 32 a according to therotational direction thereof, the throttle 32 c provided in a conduitconnecting the high pressure selector valve 32 b and the tank 19, and anintake valve 32 d for taking oil into the vehicle speed detection pump32 a from the tank 19. The hydraulic vehicle speed detecting means 32Adetects vehicle speed by vehicle speed signal pressure Pv (proportionalto vehicle speed) produced when the discharge oil of the vehicle speeddetection pump 32 a is drained via the oil pressure selector valve 32 band the throttle 32 c.

A control valve means 30A is equipped with a tilt rotation fixingcontrol valve 11B, a Hi-Lo switching solenoid valve 13B, and aslow-return valve 25. In an acceleration mode, the tilt rotation fixingcontrol valve 11B is immediately switched to position “b” at a fourthvehicle speed Ve4, and gradually switched to position “a” by theslow-return valve 25 from the point where the vehicle speed is thefourth vehicle speed Ve4 in a deceleration mode. When the tilt rotationfixing control valve 11B is at position “a”, the discharge port of acontrol pump 23 is connected to an inlet port of a maximum tilt rotationfixing valve 21A, and a pilot pressure receiving portion adjacent to thefloating end of the small tilt rotation fixing valve 13A and the pilotpressure receiving portions of the zero tilt rotation fixing valve 11Aare connected to the tank 19. When the tilt rotation fixing controlvalve 11B is at position “b”, the discharge port of a control pump 23 isconnected to the clutch 5, the pilot pressure receiving portion adjacentto the floating end of the small tilt rotation fixing valve 13A, and thepilot pressure receiving portions of the zero tilt rotation fixing valve11A, and the inlet port of the maximum tilt rotation fixing valve 21A isconnected to the tank 19.

The Hi-Lo switching solenoid valve 13B is set to Hi position when thesolenoid is deenergized by a Hi-Lo shifting switch 31, and set to Loposition when the solenoid is energized. At the Hi position, the pilotpressure receiving portion adjacent to the extension end of the smalltilt rotation fixing valve 13A and the pilot pressure receiving portionof the maximum tilt rotation fixing valve 21A are connected to the tank19. At the Lo position, the discharge port of the control pump 23 isconnected to the pilot pressure receiving portion adjacent to theextension end of the small tilt rotation fixing valve 13A and the pilotpressure receiving portion of the maximum tilt rotation fixing valve21A. The outlet port of the maximum tilt rotation fixing valve 21A isconnected to a second pilot pressure receiving portion 28 b of thesecond servo valve 28.

In the graph shown in FIG. 2A, the axis of abscissa indicates vehiclespeed, and the axis of ordinates indicates the tilt rotation amounts ofthe first and second hydraulic motors 1 and 2, respectively, in terms ofdischarge capacity (cc/rev). The graph illustrates the relationshipbetween vehicle speed and the tilt rotation amounts of the hydraulicmotors 1 and 2 in relation to the discharge volume of a hydraulic pumpwhen an accelerator of a vehicle is fully depressed from a state wherethe vehicle is at rest. FIG. 2B illustrates an operational relationshipamong the small tilt rotation fixing valve 13A, the zero tilt rotationfixing valve 11A, the tilt rotation fixing control valve 11B, and theclutch 5 in relation to vehicle speed indicated on the axis of abscissaof FIG. 2A. In the graph, acceleration is denoted by solid lines, anddeceleration is denoted by dashed lines.

The operation of the first embodiment will now be described.

(1) operation for increasing the speed of a vehicle (Low-speed rangewherein the vehicle speed is at the fourth vehicle speed Ve4 or less)

When the Hi-Lo switching solenoid valve 13B is deenergized to set it tothe Hi position, the pilot pressure receiving portion adjacent to theextension end of the small tilt rotation fixing valve 13A and the pilotpressure receiving portion of the maximum tilt rotation fixing valve 21Aare drained. The vehicle speed signal pressure Pv is low, so that thetilt rotation fixing control valve 11B is set to position “a”. Thisdrains the pilot pressure receiving portion adjacent to the floating endof the small tilt rotation fixing valve 13A and the pilot pressurereceiving portions of the zero tilt rotation fixing valve 11A. Thus, thezero tilt rotation fixing valve 11A is set in the shut-off position“a”nd the small tilt rotation fixing valve 13A is switched to a floatingposition “b”y a spring force, causing the stopper cylinder 14 to float.A clutch oil pressure is drained via position “a” of the tilt rotationfixing control valve 11B, so that the clutch 5 is connected. A constantcontrol pressure Pc is supplied to a second pilot pressure receivingportion 28 b of the second servo valve 28 via position “a” of the tiltrotation fixing control valve 11B and the Hi position of the maximumtilt rotation fixing valve 21A, and a second opening for placing port Pand port A of the second servo valve 28 placed in communication isenlarged.

When the vehicle is begin to be driven in a forward (F) mode, a drivepressure Pac of the first main circuit 17 that has been increased to ahigh pressure is supplied to the rod chamber 7 a of the first tiltrotation cylinder 7, the first pilot pressure receiving portion 8 a ofthe first servo valve 8, and port P of the first servo valve 8 viaposition F of the FR switching valve 6. The variable control pressure Pmproportional to the engine speed that is controlled by an acceleratorpedal or the like (not shown) is supplied to the second pilot pressurereceiving portion 8 b of the first servo valve 8 via position F of theFR switching valve 6 and the shuttle valve 12. The first servo valve 8is controlled to a position where the variable control pressure Pm ofthe second pilot pressure receiving portion 8 b, the drive pressure Pacof the first pilot pressure receiving portion 8 a, and the urging forceof the spring 8 c are balanced. When the drive of the vehicle isstarted:Pm<<Pac

Hence, a first opening that establishes communication between port P andport A of the first servo valve 8 becomes smaller. Thus, the drivepressure Pac of port P is reduced and the oil pressure supplied to thebottom chamber 7 b of the first tilt rotation cylinder 7 is low, causingthe first hydraulic motor 1 to have its maximum tilt rotation amount, asshown in FIG. 2A. On the other hand, a second opening of the secondservo valve 28 is enlarged, so that the drive pressure Pac of port P issupplied, without being reduced, to the bottom chamber 27 b of thesecond tilt rotation cylinder 27, causing the second hydraulic motor 2to be fixed at its maximum tilt rotation amount, as shown in FIG. 2A.Since both hydraulic motors 1 and 2 are set at the maximum tilt rotationamounts, the vehicle begins its startup at maximum torque. If theconstant control pressure Pc is set at a value larger than the maximumvalue of the variable control pressure Pm, then the outlet port of thetilt rotation fixing control valve 11B may be directly connected to theshuttle valve 12, omitting the zero tilt rotation fixing valve 11A.

At the beginning of the startup, the majority of the drive pressure Pacis mostly relieved by a relief valve (not shown) for restricting themaximum pressure of the drive pressure Pac. The relief flow, however, isgradually decreased, and the flow supplied to the hydraulic motors 1 and2 increases. This causes the vehicle speed to continue to increase up toa first vehicle speed Ve1 even if the tilt rotation amounts of thehydraulic motors 1 and 2 are constant, as shown in FIG. 2A. Thereafter,the drive torque of the vehicle gradually decreases, and the drivepressure Pac drops until the following relationship is established:(Urging force of Pm)>(Urging force of Pac)

Thus, the first opening of the first servo valve 8 is enlarged, and theoil pressure supplied from port A to the bottom chamber 7 b of the firsttilt rotation cylinder 7 increases. As a result, the drive pressure Pac,which has dropped, is fed to the rod chamber 7 a of the first piston 7,and the piston rod 7 c is moved to the left due to a difference in area,causing the tilt rotation amount (discharge capacity D) of the firsthydraulic motor 1 to decrease. At this time, the stopper cylinder 14 isfloating, so that the tilt rotation amount (discharge capacity D) of thefirst hydraulic motor 1 continues to decrease until it reaches nearlyzero even if the stopper 7 d abuts against the piston rod 14 a. When thevehicle speed signal pressure Pv supplied through the slow-return valve25 is at the fourth vehicle speed Ve4, a command pressure Pcs of thetilt rotation fixing control valve 11B reaches a zero fixing pressurePf, as shown in FIG. 2B, so that the zero tilt rotation fixing valve 11Ais switched, fixing the first hydraulic motor 1 substantially at thezero tilt rotation amount.

If the solenoid of the Hi-Lo switching solenoid valve 13B is energizedto set the Lo mode before the tilt rotation amount of the firsthydraulic motor 1 reaches a predetermined small tilt rotation amountthat is larger than the zero tilt rotation amount, then the constantcontrol pressure Pc is supplied to the pilot pressure receiving portionadjacent to the extension end of the small tilt rotation fixing valve13A and the pilot pressure receiving portion of the maximum tiltrotation fixing valve 21A. This causes the small tilt rotation fixingvalve 13A to be switched to the extension position “a”nd the stopper 7 dto abut against the piston rod 14 a that has extended to restrict thesmall tilt rotation amount of the first hydraulic motor 1. As a result,the tilt rotation amount (discharge capacity D) of the first hydraulicmotor 1 decreases to the small tilt rotation amount and fixed at thesmall tilt rotation amount, as shown in FIG. 2A. Hence, the speed willbe lower and the torque will be higher than in the Hi mode by theincrease in the tilt rotation amount (discharge capacity D) of the firsthydraulic motor 1. At the same time, since the constant control pressurePc is supplied to the second pilot pressure receiving portion 28 b ofthe second servo valve 28, the tilt rotation amount (discharge capacityD) of the second hydraulic motor 2 increases up to the maximum tiltrotation amount and fixed thereat. Therefore, the second hydraulic motor2 will also have lower speed and higher torque. Thus, the vehicle speedin the acceleration mode can be freely switched between the floatingposition “a”nd the extension position “b”y using the Hi-Lo mode signalsuntil the vehicle speed in the acceleration mode reaches the secondvehicle speed Ve2.

(2) operation for increasing the speed of the vehicle (High-speed rangewherein the vehicle speed exceeds the fourth vehicle speed Ve4)

If the vehicle speed signal pressure Pv increases to a high pressurewhile the Hi-Lo switching valve 13B and the maximum tilt rotation fixingvalve 21A are retained at the Hi position, then the tilt rotation fixingcontrol valve 11B is set to position “b”, and constant control pressurePc is supplied to the pilot pressure receiving portion adjacent to thefloating end of the small tilt rotation fixing valve 13A and the pilotpressure receiving portions of the zero tilt rotation fixing valve 11A.The pilot pressure receiving portion adjacent to the extension end ofthe small tilt rotation fixing valve 13A is drained. Hence, as shown inFIG. 2B, the zero tilt rotation fixing valve 11A is switched to thecommunication position “a”nd the small tilt rotation fixing valve 13A isswitched to the floating position, causing the stopper cylinder 14 tofloat. The second pilot pressure receiving portion 28 b of the secondservo valve 28 is drained via the Hi position of the maximum tiltrotation fixing valve 21A and position “b” of the tilt rotation fixingcontrol valve 11B. Thus, the second hydraulic motor 2 is released fromits maximum tilt rotation amount, as shown in FIG. 2B.

When the zero tilt rotation fixing valve 11A is in the communicationposition, the drive pressure Pac is supplied to the second pilotpressure receiving portion 8 b of the first servo valve 8 through theintermediary of the shuttle valve 9, the communication position of thezero tilt rotation fixing valve 11A, and the shuttle valve 12. Thepressure receiving area of the second pilot pressure receiving portion 8b is larger than the first pilot pressure receiving portion 8 a; hence,the drive pressure Pac overcomes the urging force from the drivepressure Pac of the first pilot pressure receiving portion 8 a and theurging force of the spring 8 c, and continues control until the firstopening of the first servo valve 8 reaches its maximum size. Thus, thedrive pressure Pac is fed to the rod chamber 7 a of the first tiltrotation cylinder 7, and when the drive pressure Pac is fed also to thebottom chamber 7 b of the first tilt rotation cylinder 7 from port A ofthe first servo valve 8, the piston rod 7 c is moved to the left due tothe difference in the pressure receiving area between the rod chamber 7a and the bottom chamber 7 b. At this time, since the small tiltrotation fixing valve 13A is still in the floating position, the pistonrod 14 a of the stopper cylinder 14 is pushed by the stopper 7 d of thefirst tilt rotation cylinder 7 to the nearly zero tilt rotation amount,thereby fixing the tilt rotation amount (discharge capacity) of thefirst hydraulic motor 1 substantially to zero. Thereafter, when thevehicle speed reaches a fifth vehicle speed Ve5, the command pressurePcs reaches a release pressure Pk for releasing the engagement of theclutch 5, so that the clutch is released, and the vehicle is driven onlyby the second hydraulic motor 2, as shown in FIG. 2A.

(3) operation for decreasing the speed of the vehicle (High-speed rangewherein the vehicle speed exceeds the fourth vehicle speed Ve4)

As in the case of the operation for accelerating the vehicle (high-speedrange), the Hi-Lo switching solenoid valve 13B and the maximum tiltrotation fixing valve 21A are in the Hi position, the zero tilt rotationfixing valve 11A is in the communication position “a”nd the small tiltrotation fixing valve 13A is in the floating position “a”ccordingly, thefirst hydraulic motor 1 is fixed to the nearly zero tilt rotationamount, while the second hydraulic motor 2 is free because its maximumtilt rotation amount has been cleared. Hence, a braking pressure Pbr ofthe second main circuit 18 that has increased due to the deceleration ofthe vehicle acts upon the first pilot pressure receiving portion 28 a ofthe second servo valve 28 via the shuttle valve 29, causing the secondopening of the second servo valve 28 to reach its maximum size. Thus,the oil pressure of the bottom chamber 27 b of the second tilt rotationcylinder 27 increases, and the second hydraulic motor 2 reaches itsmaximum tilt rotation amount indicated by the dashed line, as shown inFIG. 2A, causing the vehicle to decelerate at the maximum braking torqueof the second hydraulic motor 2.

In this state, if the solenoid of the Hi-Lo switching solenoid valve 13Bis energized to set the Lo mode, then constant control pressure Pc issupplied to the pilot pressure receiving portion adjacent to theextension end of the small tilt rotation fixing valve 13A and the pilotpressure receiving portion of the maximum tilt rotation fixing valve21A. Furthermore, the constant control pressure Pc is supplied also tothe pilot pressure receiving portion adjacent to the floating end of thesmall tilt rotation fixing valve 13A and the pilot pressure receivingportions of the zero tilt rotation fixing valve 11A. Hence, the smalltilt rotation fixing valve 13A is shifted to the floating position “a”ndthe stopper 7 d abuts against the extended piston rod 14 a, causing thetilt rotation amount (discharge capacity D) of the first hydraulic motor1 to decrease to the zero tilt rotation amount and fixed thereat. At thesame time, since the constant control pressure Pc is supplied to thesecond pilot pressure receiving portion 28 b of the second servo valve28, the tilt rotation amount (discharge capacity D) of the secondhydraulic motor 2 increases until it reaches the maximum tilt rotationamount and is fixed thereat. Accordingly, the second hydraulic motor 2will also have low speed and high torque. When the vehicle speed is atthe fifth vehicle speed Ve5 or less, as shown in FIG. 2B, the clutch oilpressure is drained via position “a” of the tilt rotation fixing controlvalve 11B, causing the clutch 5 to be engaged. The control valve means30A receives the vehicle speed signal pressure Pv from the hydraulicvehicle speed detecting means 32A through the intermediary of theslow-return valve 25; therefore, the vehicle speed signal pressure Pvdecreases when the pilot pressure of the tilt rotation fixing controlvalve 11B is equivalent to the fourth vehicle speed Ve4, and the vehiclespeed also decreases to a level lower than the fourth vehicle speed Ve4.The vehicle speed is a value that depends primarily upon the orificediameter of the slow-return valve 25 and the deceleration of the vehiclespeed; the vehicle speed is a third vehicle speed Ve3 in thisembodiment. If the Hi mode is shifted to the Lo mode at a vehicle speedhigher than the third vehicle speed Ve3 or more, then the tilt rotationfixing control valve 11B is immediately shifted to position “a”. Thisprocess is the same as the one described above, and will not berepeated.

(4) Operation for Decreasing the Speed of the Vehicle (Low-speed RangeWherein the Vehicle Speed is the Fourth Vehicle Speed Ve4 or Less)

When the vehicle speed is decelerated down to the third vehicle speedVe3 (the pilot pressure of the tilt rotation fixing control valve 11Bcorresponds to the fourth vehicle speed Ve4), the tilt rotation fixingcontrol valve 11B is shifted to position “a”, so that the tilt rotationamount of the first hydraulic motor 1 substantially fixed to zero iscleared, as shown in FIG. 2B. In other words, the fixing of the tiltrotation amount of the first hydraulic motor 1 to nearly zero remainscleared up to the fourth vehicle speed Ve4 in the acceleration mode,while it remains cleared up to the third vehicle speed Ve3 in thedeceleration mode. Thus, control hunting can be prevented by using thefourth vehicle speed Ve4 employed for switching from unfixing to fixingof the zero tilt rotation amount in the acceleration mode, and the thirdvehicle speed Ve3 employed for switching from fixing to unfixing of thezero tilt rotation amount in the deceleration mode. Furthermore, thetilt rotation amount of the first hydraulic motor 1 at a point (thethird vehicle speed Ve3) where fixing of the tilt rotation amount of thefirst hydraulic motor 1 to substantially zero in the deceleration modeis smaller than in the acceleration mode is cleared, and the brakingtorque decreases. Hence, a deceleration shock can be prevented even ifthe acceleration performance is improved. In place of electroniccontrol, hydraulic control can be conducted, permitting improved freedomin selecting a control unit. Thus, when the zero tilt rotation fixingvalve 11A is set in the shut-off position, the small tilt rotationfixing valve 13A is switched to the floating position “b”y the springforce, causing the stopper cylinder 14 to float. After the fixing forsetting the first hydraulic motor 1 to nearly the zero tilt rotationamount is cleared, the drive pressure Pac starts to increase as thevehicle speed decreases, so that the tilt rotation amount of the firsthydraulic motor 1 increases from the small tilt rotation amount to themaximum tilt rotation amount, as shown in FIG. 2A. Hence, even when thesecond hydraulic motor 2 remains at the maximum tilt rotation amount,the vehicle speed decreases as the tilt rotation amount (dischargecapacity D) of the first hydraulic motor 1 increases. When the vehiclespeed drops down to the first vehicle speed Ve1 or less, both first andsecond hydraulic motors 1 and 2 reach maximum tilt rotations; however,the amount of oil relieved from a relief valve (not shown) installed inthe main circuit increases, causing the vehicle to further reduce itsspeed.

Vehicle speeds Ve1, Ve2, Ve3, Ve4, Ve5, and Ve6 in this embodimentincrease in the ascending order, the first vehicle speed Ve1 being thelowest speed, and the sixth vehicle speed Ve6 being the highest speed.

The operation and advantage of the first embodiment will now bedescribed in conjunction with FIG. 3B. FIG. 3A shows a relationshipamong time t, vehicle speed V, and command pressure Pcs in theacceleration mode, and FIG. 3B shows a relationship among time t,vehicle speed V, and command pressure Pcs in the deceleration mode.

The vehicle speed signal pressure Pv increases in the acceleration mode,and when the force acting upon the pressure receiving portion of thetilt rotation fixing control valve 11B becomes larger than the urgingforce of the spring of the tilt rotation fixing control valve 11B, thetilt rotation fixing control valve 11B starts to shift from position “a”to position “b”, and the command pressure Pcs starts to increase (timeta in FIG. 3A). The vehicle speed signal pressure Pv at that point isdefined as a start pressure Pb.

Until the vehicle speed signal pressure Pv reaches the start pressure Pbof a predetermined value, the command pressure Pcs output is released atthe return pressure Pt. The return pressure Pt is the oil pressure of acircuit in communication with the tank 19.

As the vehicle speed V increases and the vehicle speed signal pressurePv increases accordingly, the command pressure Pcs increases on thebasis of the movement of a spool of the tilt rotation fixing controlvalve 11B toward position “b”. When the vehicle speed signal pressure Pvreaches a value equivalent to the fourth vehicle speed Ve4, the commandpressure Pcs reaches the zero fixing pressure Pf. When the spool of thetilt rotation fixing control valve 11B fully moves to position “b” viathe release pressure Pk, the command pressure Pcs turns into theconstant control pressure Pc.

In the deceleration mode, when the vehicle speed decreases and reachesthe sixth vehicle speed Ve6 (time td in FIG. 3B), the tilt rotationfixing control valve 11B begins to shift from position “b” to position“a”. In this case, the command pressure Pcs is the constant controlpressure Pc at position “b”, approaches to the zero value as position“a” is closer, and reaches the zero value when position “a” is reached.The switching from position “b” to position “a” is gradually performedbecause the oil of the pressure receiving portion of the tilt rotationfixing control valve 11B is returned through the intermediary of theslow-return valve 25.

The urging force of a spring 82 of the zero tilt rotation fixing valve11A is set such that the zero tilt rotation fixing valve 11A is switchedfrom the shut-off position to the communication position when thecommand pressure Pcs is larger than the zero fixing pressure Pf forfixing to the zero tilt rotation amount. The urging force of the spring72 of the clutch 5 is set such that the clutch 5 is released fromengagement when the command pressure Pcs reaches the release pressure Pkof the clutch 5 that is higher than the zero fixing pressure Pf.

In the acceleration mode, the tilt rotation amount of the swash plateangle of the first hydraulic motor 1 is not restrained until the vehiclespeed V reaches the fourth vehicle speed Ve4, and the clutch 5 is inengagement, the vehicle body being driven by both the first and secondhydraulic motors 1 and 2. When the vehicle speed V reaches the fourthvehicle speed Ve4, the command pressure Pcs reaches the zero fixingpressure Pf, and the zero tilt rotation fixing valve 11A is set at thecommunication position, fixing the first hydraulic motor 1 to the zerotilt rotation amount. As the vehicle speed V further increases until itreaches the fifth vehicle speed Ve5, the command pressure Pcs reachesthe release pressure Pk, disengaging the clutch 5. Thus, the vehiclebody is driven only by the second hydraulic motor 2.

In the deceleration mode, the command pressure Pcs decreases, and theoil in an oil chamber 73 of the clutch 5 is set in communication withthe return pressure Pt through the throttle at position “a” of the tiltrotation fixing control valve 11B. The shift from position “b” toposition “a” is gradually performed by the throttle o the slow-returnvalve 25; therefore, the oil pressure of the oil chamber 73 is graduallydecreased. This causes the clutch 5 to be engaged without a shock. Whenthe command pressure Pcs drops down to the release pressure Pk, theengagement of the clutch 5 is completed. As the command pressure Pcs isfurther decreased until it reaches the zero fixing pressure Pf, thefixing of the zero tilt rotation amount is cleared.

Thus, when the clutch is released in the acceleration mode, the firsthydraulic motor 1 is securely fixed to the zero tilt rotation amount,and when the clutch 5 is engaged in the deceleration mode, the firsthydraulic motor 1 has always been fixed to the zero tilt rotationamount. This feature enables reliable prevention of a speed change shockor load slip in the first hydraulic motor 1.

Furthermore, the speed at which the operation position of the tiltrotation fixing control valve 11B is shifted in the deceleration modecan be easily set by changing the sizes of the throttles at position “a”of the slow-return valve 25 and the tilt rotation fixing control valve11B. This makes it possible to arbitrarily set a time gradient of thecommand pressure Pcs of the tilt rotation fixing control valve 11B,permitting the clutch 5 to be engaged further smoothly. Moreover, thepressure receiving areas of the valves and the urging forces of thesprings are set so that the zero fixing pressure Pf is smaller than therelease pressure Pk so as to make sure that the fixing of the zero tiltrotation is cleared after the clutch 5 has been engaged. These featuresmake it possible to provide an apparatus for controlling a plurality ofhydraulic motors 1, 2 and the clutch 5 that features outstandinglysmooth operation.

A second embodiment will now be described with reference to FIG. 4. Onlythe aspect that is different from the first embodiment shown in FIG. 1will be described, and the same components will be designated by thesame reference numerals and the description thereof will be omitted.

Referring to FIG. 4, the zero tilt rotation fixing valve 11A and theshuttle valve 12 in the first embodiment shown in FIG. 1 are omitted,and a pressure receiving portion 8 d is provided adjacent to a pressurereceiving portion 8 b of a first servo valve 8, an outlet port of a tiltrotation fixing control valve 11B being connected to the pressurereceiving portion 8 d of the first servo valve 8. A zero fixing pressurePf for fixing the swash plate angle of the first hydraulic motor 1 tothe zero tilt rotation amount is set to be smaller than a releasepressure Pk at which the engagement of a clutch 5 is released.

In an acceleration mode, as vehicle speed V increases and when the tiltrotation fixing control valve 11B is actuated at position “b”, aconstant control pressure Pc is supplied to the pressure receivingportion 8 d of the first servo valve 8. This causes a drive pressure Pacto be supplied to a bottom chamber 7 b of a first tilt rotation cylinder7 through the intermediary of an FR switching valve 6 and the firstservo valve 8, so that a piston rod 7 c is moved to the left, therebyfixing a zero tilt rotation amount. Thereafter, the clutch 5 isdisengaged.

Thus, a sequence for engaging and disengaging the clutch 5 and forfixing and clearing the zero tilt rotation amount of a first hydraulicmotor 1 can be always reliably implemented simply by switching the tiltrotation fixing control valve 11B. This permits reliable prevention of aspeed change shock or load slip in the first hydraulic motor 1.

A third embodiment will now be described in conjunction with FIG. 5.Only the aspect that is different from the first embodiment shown inFIG. 1 will be described, and the same components will be designated bythe same reference numerals and the description thereof will be omitted.

Referring to FIG. 5, the zero tilt rotation fixing valve 11A and theshuttle valve 12 in the first embodiment shown in FIG. 1 are omitted,and a pressure receiving portion 8 d is provided adjacent to a pressurereceiving portion 8 b of a first servo valve 8, an outlet port of a tiltrotation fixing control valve 11B being connected to the pressurereceiving portion 8 d of the first servo valve 8.

Furthermore, a zero tilt rotation detecting valve 80 is provided thatdetects the presence of a contracting end of a piston rod 14 a in thebottom chamber of a stopper cylinder 14, and supplies a command pressurePcs to an oil chamber 73 of a clutch 5. When the piston rod 14 a is atits contraction end, the zero tilt rotation detecting valve 80 isactuated at position “b”, causing the command pressure Pcs to besupplied to the oil chamber 73 of the clutch 5 via position “b” of thetilt rotation fixing control valve 11B and position “b” of the zero tiltrotation detecting valve 80.

In an acceleration mode, as vehicle speed V increases and when the tiltrotation fixing control valve 11B is actuated at position “b”, a commandpressure Pcs is supplied to the pressure receiving portion 8 d of thefirst servo valve 8. This causes a drive pressure Pac to be supplied toa bottom chamber 7 b of a first tilt rotation cylinder 7 through theintermediary of an FR switching valve 6 and a first servo valve 8, sothat a piston rod 7 c is moved to the left, thereby fixing a zero tiltrotation amount. Then, the zero tilt rotation detecting valve 80 detectsthe zero tilt rotation amount, and the zero tilt rotation detectingvalve 80 is actuated at position “b”, causing the command pressure Pcsto be supplied to the oil chamber 73 of the clutch 5 through theintermediary of position “b” of the tilt rotation fixing control valve11B and a check valve 81, thus disengaging the clutch 5.

As described above, a hydraulic circuit for releasing the engagement ofthe clutch 5 is actuated after the zero tilt rotation detecting valve 80mechanically detects that a first hydraulic motor 1 has reached the zerotilt rotation amount. With this arrangement, a sequence for engaging anddisengaging the clutch 5 and for fixing and clearing the zero tiltrotation amount of the first hydraulic motor 1 can be always reliablyimplemented, permitting reliable prevention of a speed change shock orload slip in the first hydraulic motor 1.

Thus, according to the present invention, the springs and pressurereceiving areas of valves are set such that the magnitude of the releasepressure for disengaging a clutch is larger than a fixing pressure forfixing the zero tilt rotation amount of a hydraulic motor. The commandpressure from the tilt rotation fixing control valve of a tilt rotationfixing means at which the operation position is switched by a vehiclespeed signal pressure is output as a pilot pressure for a zero tiltrotation fixing means for fixing to zero tilt rotation or a servo valvethat controls the tilt rotation amount, and the hydraulic motor is fixedto the zero tilt rotation amount when a vehicle speed is higher than apredetermined vehicle speed. In the case of a control circuit providedwith a zero tilt rotation detecting valve, the command pressure of thetilt rotation fixing control valve is supplied to the oil chamber of theclutch via the zero tilt rotation detecting valve. With thisarrangement, the hydraulic motor is fixed to the zero tilt rotationamount before the clutch is released in the acceleration mode. When theclutch is engaged from the released state in the deceleration mode, thehydraulic motor maintains the zero tilt rotation amount, and thereafter,the fixing to the zero tilt rotation amount is cleared. Thus, wheneverthe clutch is released, the hydraulic motor is always set to the zerotilt rotation amount, making it possible to accomplish an apparatus forcontrolling a plurality of hydraulic motors and a clutch that is free ofa speed change shock and also of load slip.

Moreover, the engagement and disengagement of the clutch and the fixingand unfixing of the zero tilt rotation are controlled simply by thecommand pressure of the tilt rotation fixing control valve. This featureensures reliable implementation of the sequence and also simplifies thecontrol circuit, contributing to lower cost.

In addition, the speed at which the switching between the positions atwhich the tilt rotation fixing control valve is actuated in thedeceleration mode can be easily set by changing the sizes of thethrottles of a slow-return valve and the tilt rotation fixing controlvalve. This allows the time gradient of the command pressure Pcs to bearbitrarily set, so that the clutch engaging time can be set for eachvehicle so that there will be no shock. In this case also, the fixing tothe zero tilt rotation is cleared only after the clutch has beenengaged. These features make it possible to provide an apparatus forcontrolling a plurality of hydraulic motors and a clutch that featuresoutstandingly smooth operation.

1. An apparatus for controlling a plurality of hydraulic motors and aclutch in which a single driving shaft is driven by outputs of aplurality of hydraulic motors, and one of the plurality of hydraulicmotors drives the driving shaft through the clutch, comprising: a firstservo valve that controls the tilt rotation amount of a first hydraulicmotor, and sets the tilt rotation amount of the first hydraulic motor toa zero tilt rotation amount when a zero fixing pressure (Pcs=Pf) of apredetermined value is input, wherein the clutch is disengaged when arelease pressure (Pk) of a predetermined value that is larger than thezero fixing pressure (Pf) of the predetermined value is input; hydraulicvehicle speed detecting means for detecting a vehicle speed by a vehiclespeed signal pressure (Pv) based on a vehicle speed; and control valvemeans that releases an output command pressure (Pcs) to a returnpressure (Pt) connected to a tank until a vehicle speed signal pressure(Pv) received from the hydraulic vehicle speed detecting means reaches astart pressure (Pb) of a predetermined value, and begins to output thecommand pressure (Pcs) to the first servo valve and the clutch when thevehicle speed signal pressure (Pv) exceeds the start pressure (Pb) ofthe predetermined value.
 2. An apparatus for controlling a plurality ofhydraulic motors and a clutch according to claim 1, further comprising:a zero tilt rotation detecting valve that detects the tilt rotationamount of the first hydraulic motor, and causes a command pressure (Pcs)to be in communication with the clutch to disengage the clutch when thezero tilt rotation amount is detected, wherein the control value meansthe command pressure (Pcs) to the first servo valve and the zero tiltrotation detecting valve when the vehicle speed signal pressure (Pv)exceeds the start pressure (Pb) of the predetermined value.